linux-kernel - Re: __weak vs ifdef

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Message-ID: <alpine.LFD.2.00.1002171819020.4141@localhost.localdomain>
Date:	Wed, 17 Feb 2010 18:22:29 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Grant Likely <grant.likely@...retlab.ca>
cc:	Matthew Wilcox <matthew@....cx>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Adrian Bunk <bunk@...nel.org>,
	Andrea Righi <righi.andrea@...il.com>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: __weak vs ifdef

On Wed, 17 Feb 2010, Grant Likely wrote:
> 
> Question.  If I use this pattern, and use the __weak attribute on core
> code functions wrapped with a #ifndef, then how does it mesh with
> EXPORT_SYMBOL*() statements?

You can't. Or at least not the traditional way, which is to put the 
EXPORT_SYMBOL next to the definition of what gets exported.

If you use __weak, you need to make sure that all users of said weak 
symbol are in another file. There are some compiler and/or binutils bugs 
with using weak symbols in the same translation unit, so the rule is that 
a weak definition hes to be somewhere else from the use - including 
EXPORT_SYMBOL.

> I also assume that at the core code site, the EXPORT_SYMBOL() must
> appear inside the #ifndef block so that a #define override doesn't
> break things.  Correct?

See above. You can't put it inside the _same_ #ifndef block. You'd have to 
put it in a different file, but yes, inside an ifndef. At which point the 
linker will just pick the right definition.

However, in general, this probably gets ugly enough that the whole __weak 
thing is not great for exported stuff. It's likely mostly useful for some 
"generic" arch functionality that is always compiled in.

		Linus
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